WALLACE RESOURCE LIBRARY Module 03 – Animal behaviour: D03 – Howler monkey vocalisation

Howler monkey 15 species currently recognised Mantled howler (Alouatta palliata) Named because of their long ‘guard’ hairs Loud territorial vocalisations Mantled howler monkeys are large bodied primates that are brown/black in colouration with a lighter coloured main or mantle along the shoulders and back which varies from red to golden in colour. Howler monkeys are diurnal group-living primates that live in multi-male-multi-female groups of up to 40 individuals and have a geographical range that incorporates southern Mexico, Guatemala, Honduras and Costa Rica. Howler monkeys feed primarily on mature leaves, but will eat young leaves, shoots, flowers and fruit when available. They have a special digestive system that enables them to break down toxins found in mature leaves using fermentation digestion. Howler monkeys are therefore able to cope with disturbed habitats and reductions in food supply as they can eat almost anything in the forest, whereas other primates have more specialised diets. As a result of the high-toxin load in their diet and the energy requirement of fermentation digestion, howler monkeys are famously inactive and spend up to 80% of their day resting. Howler monkeys take their name from their territorial loud call vocalizations that can be heard from distances of up to 3km. Howler monkeys generally make these loud calls at dawn each day and neighbouring groups will respond to create a deafening noise in the forest.

Tasks for the data set Listen to all of the howler monkey calls and see if you can identify how many males are calling Analyse behavioural responses of hower monkeys Data collection methods: The study was conducted at Rancho Manacal, which is a privately owned sugar cane plantation and cattle ranch with adjacent forest patches located outside the town of Cofradia at the base of the Merendon Mountains. There are eight different groups of mantled howler monkeys (Alouatta palliata) in this tiny forest fragment producing an extremely high density of 232 individuals per km2. Behavioural observations were conducted on two of the eight groups per day for 6 or 7-hour periods either from 05.00 until 12.00 or from 12.00 until 18.00. An equal number of morning and afternoon observations were conducted for each group. Vocalizations, specifically male loud calls, were recorded ad libitum, noting the time of the call, the number of males calling, the origin of the loud call (whether it was made by males from the group being observed, or from a different group), and the number of the group that made the call. On the initiation of a loud call, the behaviour (feeding, moving, resting, socializing or vigilant) of all of the females with infants (mothers) and females without infants (non-mothers) in view were recorded using one instantaneous scan (rapidly scanning from the left to the right of the group and noting down the behaviour of each female in view at the exact time that the loud call was made). Analysis methods: Howler monkeys use loud calls to assess the numerical odds of other groups and determine their vulnerability to group take over. If you listen to loud calls carefully, we can also determine the number of males that are calling because the males join the calling bout a few split seconds after each other. NOW OPEN THE FOLDER “Howler_Monkey_Call_Library” (found in the folder 4. Data for task') Listen to each of the calls in the folder and count how many males are calling in each. It is not possible to recognise all the different individual monkeys in each group at Manacal so we cannot analyse the data at the individual level. That means that the data must be summarised for each of the eight groups and then an average value in calculated across groups. NOW OPEN THE FILE “M03 D03 EXAMPLE” The file contains the frequency of each type of behavioural response (feeding, moving, resting, socializing, or vigilant) to loud calls made by females without infants (non-mothers) and females with infants (mothers). The behavioural response we are specifically interested in is vigilance because this indicates that the females were concerned by the loud calls. What we want to know is whether mothers were vigilant in response to loud calls more often that non-mothers. To do this we need to calculate the proportion of mother and non-mother responses that were vigilant for each of the eight groups of howlers. In the table called “non-mother responses to loud calls made my neighbouring groups” you will see an empty column for the proportion of responses that were vigilant. You need to calculate the frequency of vigilance responses divided by the total number of responses made by non-mothers in group 1 and you can do this by typing the formula =F4/G4 and then enter. This formula tells excel to divide the number in cell F4 (frequency of vigilance) by the number in cell G4 (total frequency of responses). Now that you have calculated this proportion for non-mothers in group 1 you need to calculate the same value for non-mothers in all the other groups. The easiest way to do this is to use the click and drag function in Excel. Click on the value you have just calculated (i.e. click on cell H4), and move the cursor to the bottom right of the cell so that the cross turns from white to black. Click on the left button of your mouse and drag the cursor down the column at the same time. Excel should then calculate the proportion of non-mother responses that were vigilant for each of the remaining groups of howlers. You now have the proportion of responses that were vigilant for non-mothers, but we cannot directly compare these values to those of non-mothers because there were different numbers of mothers and non-mothers in each group. We therefore need to calculate a “weighted proportion”. This involves dividing the calculated proportion for a given group by the number of mothers or non-mothers in that group. You can do this using the same sort of formula that you used previously. Click on the first empty cell in the column for weighted proportion of vigilance and type the formula =H4/I4 and then press enter. You can then use the click and drag function to complete these values for the rest of the column. The last step is to calculate an average weighted proportion of vigilance responses across all the groups. To do this, click on the cell labelled “Average Weighted Proportion of Non-Mother Vigilance” and type in the formula =average(J4:J11) and press enter. Repeat this process for the table called “mother responses to loud calls made my neighbouring groups” IF YOU OPEN THE FILE “M03 D03 COMPLETED” YOU CAN CHECK THAT YOUR CALCULATIONS ARE ALL CORRECT Now you need to find a suitable way of displaying your data so you can see if there is significant difference in the weighted proportion of vigilance responses made by mothers compared to non-mothers. This will tell us if females with infants are more concerned by loud calls than females with infants. The best way to show this is using a column graph. To create the column graph, you first need to create a summary table that can be used to create the graph. In the file Completed_Example_Female_Responses_to_Loud_Calls.xls you will see a summary for graph table. Highlight the information in this table and click on “Insert” and then “Column”, Excel will create a graph of the highlighted data. You will now want to make the figure more presentable by adding titles to both the x-axis and the y-axis, and a title to the whole graph (beginning with “Figure 1: XXXXXX). It is also advisable to delete the legend and gridlines to make the data clearer. IF YOU OPEN THE FILE “M03 D03 GRAPH” YOU WILL SEE WHAT THE FINISHED GRAPH SHOULD LOOK LIKE

Research questions: Howler monkeys can determine how many males are in each group based on the number of males that join in the loud calls made by their group. Can you correctly identify the number of males in each group based on their calls? Are females with infants more likely to be vigilant in response to loud calls than females without infants?

Summary and conclusions How many males were calling? Females with infants pay more attention to loud calls Females assess their groups vulnerability through the loud calls 1.Call a = 1 male, Call b = 3 males , Call c = 2 males, Call d = 2 males, Call e = 1 male, Call d = 2 males, Call e = 1 male, Call f = 3 males, Call g = 2 males, Call h = 1 male 2.Our data indicates that female mantled howler monkeys that have infants are significantly more likely to pay attention to loud calls made be extra-group males than females that do not have infants. This finding supports the theory that females use information from loud calls to assess their group’s vulnerability to group takeover, and the corresponding risk of infanticide associated with group take over. As females without infants would not be affected by infanticidal males, this explains why they do not really pay much attention to loud calls. However, females with infants are not always vigilant when they hear loud calls. The likely explanation for this is that females are only concerned about calls that indicate a real threat to them. In other words, they are more likely to respond when they hear more males calling than they have in their own group as this means that should a takeover occur, they would be outnumbered. Our subsequent analyses of the Manacal data has supported this theory as females with infants are significantly more likely to be vigilant when the number of calling males outnumbered the number of males in their own group.