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Re: Nocturnal crocs?



John,

The answer isn't necessarily black and white. All modern crocodylians are
considered nocturnal, but most of them are also quite active during daylight
hours. This begs the oft-asked question "do they ever sleep?", to which the
answer is yes, primarily for short periods during daylight hours, but it is
a rapidly-reversible state leading quickly to high levels of activity if
necessary (usually an escape response). Activity is also significantly
influenced by ambient temperature, and those species in temperate climates
or dry season tropical climates tend to be more active during daylight
hours, retreating into the water or burrows during the cool night. Most
foraging occurs at night when temperatures are favourable.

Approaching a crocodile nest during daylight hours can be dangerous,
although some females exhibit a propensity to flee (at least from large,
crashing humanoids with stout oars). Approaching the same nest at night
usually elicts a far more aggressive response from the female, although this
depends on temperature and I honestly wouldn't like to say whether there's
any real pattern. I am not aware of anyone who has tried objectively to
correlate nest defence activity with time of day.

There is also a lot of inter-specific variation - some defend their nest
with considerable vigour at all times (eg. Cuban crocs) whereas others (eg.
Australian freshwater crocodiles) flee at the slightest provocation (eg. a
snapping twig). The latter has been hypothesised to be exacerbated by
aboriginal subsistence hunting, leading to selection for wary crocs at the
nest.

In summary, the general trend is that nesting crocodylians are vigilant
nearly 24 hours a day. Females of most species cease hunting and feeding
behaviour to remain around the nest at all times, usually constructing a
wallow from which to access the nest quickly if necessary. Defence response
may vary with daylight, but I'm not aware of any studies.

A small caveat, however - I wouldn't exclusively describe crocodylians as
non-concealing nesters. It depends on what nesting predators exist. The
primitive nesting condition for crocs is excavating a hole in sand, lay the
eggs, and cover the area over. Visually, they are extremely difficult to
locate. Only predators sensitive to odour have any real chance of finding
one, and even then they are not 100% effective (or there would be no crocs
left in many areas today). The derived nesting strategy is to build a mound
nest, primarily to counter flooding (which for most species is a greater
threat to eggs than predation). Many species build fairly conspicuous
mounds, but a few species (eg. dwarf caiman, dwarf crocodile) nest in
heavily vegetated forest and their nests are very hard for visual predators
to locate. Perhaps odiferous vegetation (used to construct the nest) may
also throw a predator off the scent, whereas sand appears to retain scent
easily.

It doesn't strike me as difficult to construct a scenario where crocodylians
cope with mammalian (and reptilian) nest predators. They've been doing a
pretty good job for quite a while now.

I'll defer to Chris Brochu when it comes to sclerotic rings in fossil crocs,
but I've not heard of any myself. No modern species have any of course.

Adam Britton


----- Original Message -----
From: "John Bois" <jbois@umd5.umd.edu>
To: <dinosaur@usc.edu>
Sent: Friday, August 24, 2001 8:04 AM
Subject: Nocturnal crocs?


> As many good people of this list may know, I am investigating the
> possibility that relaxation on mammal size constraints toward the very end
> of the Cretaceous, brought them over the dinosaur nest/juvenile predation
> threshold.  One of the many criticisms leveled at this hypothesis is the
> following: if mammals were such a problem for non-concealing dinos, why
> didn't they take out non-concealing crocs as well?  A prediction for this
> hypothesis is that there should be some critical  difference between these
> archosauran clades.  It seems likely that most dinos were diurnal
> (see HP Rowe's paper at _Palaeontologia Electronica_ Vol 3, Issue 1).  It
> seems likely that most/many mammals were nocturnal.  Small jackals today
> cannot approach ostrich nests in the day time but have no trouble at
> night.  But what about crocs.  A prediction of this hypothesis would be
> that most/all crocs making it through the Cretaceous would be nocturnal
> (i.e., they survived because they could defend their nests at
> night).  Paul Willis seemed to feel that no Cenozoic crocs possessed
> sclerotic rings (a structure linked by Mickey and others to the diurnal
> habit).  I searched for
> about three days and could find none.  But I did find this from Underwood
> writing in _Biology of the Reptilia Vol 2.  "It may be significant that
> the only (crocodilians) that survived the Cretaceous were nocturnal."  So,
> I just wanted to see if this agreed with what this esteemed body
> knows; and, if so, whether it would be considered a nice piece of
> supporting evidence.
>